1. Field of the Invention
The present invention relates to a multilayer band-pass filter.
2. Description of the Related Art
High-frequency band-pass filters appropriate for size reduction and low profile have been configured by providing multiple LC resonators in multilayer bodies in which dielectric layers are stacked. FIG. 12 shows an example of the structure of a typical band-pass filter in the related art.
Referring to FIG. 12, a band-pass filter 1 includes multiple dielectric layers R2 to R9 and three LC resonators Q10, Q20, and Q30.
Resonator capacitor electrodes 16a, 16b, 17a, 17b, 18a, and 18b, adjacent coupling capacitor electrode R13 to R15, 19, and 20, a jump-coupling capacitor electrode 21, ground-side capacitor electrodes 31 to 33, a ground electrode 25, line electrodes 26 to 28, inductor via electrodes 10a to 10d, 11a to 11d, and 12a to 12d that penetrate through the dielectric layers, and connection via electrodes 41a, 41b, 42a, 42b, 43a, and 43b are appropriately provided on or in the dielectric layers R2 to R9.
The LC resonator Q10 includes the resonator capacitor electrodes 16a and 16b, the ground-side capacitor electrode 31, the ground electrode 25, the line electrode 26, the inductor via electrodes 10a to 10d, and the connection via electrodes 41a and 41b. 
The LC resonator Q20 includes the resonator capacitor electrodes 17a and 17b, the ground-side capacitor electrode 32, the ground electrode 25, the line electrode 27, the inductor via electrodes 11a to 11d, and the connection via electrodes 42a and 42b. 
The LC resonator Q30 includes the resonator capacitor electrodes 18a and 18b, the ground-side capacitor electrode 33, the ground electrode 25, the line electrode 28, the inductor via electrodes 12a to 12d, and the connection via electrodes 43a and 43b. 
The structure of the LC resonator Q10, which is representative of the LC resonators Q10 to Q30, will now be described in detail.
The inductor via electrodes 10a to 10d are connected to each other to define a columnar inductor L10. The columnar inductor L10 is connected to the line electrode to define the resonator inductor L10. The resonator capacitor electrodes 16a and 16b are electrically connected to each other via the connection via electrodes 41a and 41b. The resonator capacitor electrodes 16a and 16b oppose the ground electrode 25 and the ground-side capacitor electrode 31 to define a resonator capacitor C10 between the resonator capacitor electrodes 16a and 16b, the ground electrode 25, and the ground-side capacitor electrode 31. The resonator capacitor electrodes 16a and 16b are connected to the inductor via electrode 10a, which is an end of the inductor L10, and the resonator inductor L10 and the resonator capacitor C10 define the LC resonator Q10. The LC resonators Q20 and Q30 have structures similar to that of the LC resonator Q10.
The adjacent coupling capacitor electrodes R13, R14, and R15 form a capacitance with the adjacent coupling capacitor electrodes 19 and 20 to define adjacent coupling capacitors Cs10 and Cs20 for coupling between the adjacent LC resonators. The adjacent coupling capacitor electrodes R13 and R15 and the jump-coupling capacitor electrode 21 opposing the adjacent coupling capacitor electrodes R13 and R15 form a capacitance to define a coupling capacitor Cs30 for the jump-coupling between the LC resonator Q10 and the LC resonator Q30.
Since the three LC resonators defining the band-pass filter are covered with the ground electrode 25, which is disposed substantially over the dielectric layer on one main surface in the stacking direction of the multilayer body, in the band-pass filter having the above structure, it is possible to reduce noise from an external electronic component, thereby achieving excellent frequency characteristics.
For example, refer to Japanese Unexamined Patent Application Publication No. 2002-76807.
In the band-pass filter 1 described above, the ground electrode 25 disposed on the upper surface of the filter allows the external noise to be blocked to achieve the excellent frequency characteristics. However, further promotion of the size reduction and the low profile of the band-pass filter 1 decreases the thickness of the dielectric layers which decreases the distance between the ground electrode 25 and the LC resonators Q10 to Q30 defining the band-pass filter 1. This causes a problem in that unintended electromagnetic field coupling occurs between the resonator capacitor electrodes 16a, 17a, and 18a via the ground electrode 25 to prevent desired frequency characteristics from being achieved.